Porous metal structure

ABSTRACT

An open-pore polyurethane structure containing powdered metal comprising coherent spherical particles separated by interconnected interstices and a method of producing this structure comprising mixing metal powder with the components to make a polyurethane structure in a container, polymerizing the mixture in place without stirring after onset of gelation. The polyurethane can be removed preferably by heating in air at a temperature below the sintering temperature for the metal, and the remaining metal can then be sintered forming a sintered porous metal or metal oxide structure depending on the metal used and sintering conditions. A number of porous nickel products were made by the process of the invention and after the removal of the polyurethane depending on the particle size of the nickel and the amount and conditions of sintering densities of the products ranged from 1 to 5 g./cc., compressive strengths from 100 to 20,000 and porosity from 20 to 80 percent. The porous metals or sintered porous metals can be used in mechanical support, air and liquid filters, porous bearings, porous electrodes, acoustic filters, impact absorbers, capillary wicks, void fillers, integral conductor-insulator rods, and three-dimensional highmodulus reinforcements.

United States Patent 1191 Salyer et al.

[ July 29, 1975 POROUS METAL STRUCTURE [75] Inventors: lval O. Salyer,Dayton, Ohio;

Robert T. Jefferson, Columbus, Ind.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

22 Filed: Sept. 30, 1971 21 Appl. No.: 185,002

Related U.S. Application Data [63] Continuation of Ser. No. 54,298, July13, 1970, Pat. No. 3,647,721, which is a continuation-in-part of Ser.Nos. 586,923, Oct. 17, 1966, abandoned, and Ser. No. 828,647, May 28,1969, Pat. No. 3,574,150.

[52] U.S. Cl. 29/l9l.2; 29/192 R; 75/20 F;

260/25 AK; 264/44 [51] Int. Cl B23p 3/00 [58] Field of Search 29/182 R,182.1, 182.2,

29/l91.2, 192 R, 192 CP; 260/25 AK, 2.5 AX, 2.5 BE, 37 R, 77.5 R, 77.5AQ; 161/190; 106/41, 122; 264/44; 75/20 F Primary Examiner -Walter R.Satterfield Attorney, Agent,- or FirmBruce Stevens [57] ABSTRACT Anopen-pore polyurethane structure containing powdered metal comprisingcoherent spherical particles separated by interconnected interstices anda method of producing this structure comprising mixing metal powder withthe components to make a polyurethane structure in a container,polymerizing the mixture in place without stirring after onset ofgelation. The polyurethane can be, removed preferably by heating in airat a temperature below the sintering temperature for the metal, andtheremaining metal can then be sintered forming a sintered porous metal ormetal oxide structure depending on the metal used and sinteringconditions. A number of porous nickel products were made by the processof the invention and after the removal of the polyurethane depending onthe particle size of the nickel and the amount and conditions ofsintering densities of the products ranged from 1 to 5 g./cc.,compressive strengths from 100 to 20,000 and porosity from .20 to 80percent. The porous metals or sintered porous metals can be used inmechanical sup port, air and liquid filters, porous bearings, porouselectrodes, acoustic filters, impact absorbers, capillary wicks, voidfillers,,integral conductor-insulator rods, and three-dimensionalhigh-modulus reinforcements.

1 Claim, 1 Drawing Figure POROUS METAL STRUCTURE CROSS-REFERENCE TORELATED APPLICATIONS This application is a continuation of copendingapplication Ser. No. 54,298, now US. Pat. No. 3,647,721, filed July 13,1970 which is a continuation-in-part of our application Ser. No.586,923, filed Oct. 17, 1966, now abandoned, and Ser. No. 828,647 filedMay 28, 1969, now U.S. Pat. No. 3,574,150.

The invention described herein was made in the course of, or under, acontract with the United States Atomic Energy Comission.

BACKGROUND OF THE INVENTION The process of this invention is amodification of the process described in copending application Ser. No.828,647 filed May 28, 1969, in that in the present process metal powderis added to the ingredients used in the copending application to makethe polyurethane structure. A process is described in [1.8. Pat. No.3,111,396 for saturating an open-pore polyurethane foam with a slurry ofmetal powder. The compositions of the patent and those of the presentinvention, although both are polyurethane containing metal, are ofdifferent structure, the patent being a foam structure versus discreteinterconnected particles of the present invention. Both themetalcontaining polyurethanes of the patent and those of the presentapplication can be used to make porous metal articles by decomposing thepolyurethane and porous sintered metals can be made after thepolyurethane removal; however, porous metal and porous sintered metalsmade by the process of the invention are denser due to the fact moremetal can be encorporated per unit volume of polyurethane by the processof the present invention. Other advantages are also evident for theprocess of the invention in making its metalcontaining polyurethanesversus the process of the patent in that it is quite clear that it ismuch easier to encorporate the metal in the process of the presentinvention this is quite evident when comparing the teachings of the twoprocesses. Also the metal powder is more uniformly distributed in thepolyurethane by the process of the invention; and the porous metalproducts made by decomposing the polyurethane and the sintered metalproducts are denser and more uni form when made by the process of thepresent invention as compared to that of the patent. Furthermore, theporous metal products and porous sintered metal products can be made inany form by the process of the present invention since they take theshape of whatever vessel they are formed in, whereas, porous metal andporous sintered metal products made by conventional techniques whereinpowdered metal is pressed and sintered are not readily made in any shapeexcept a bar or cube shape.

SUMMARY OF THE INVENTION An object of the invention is to provide aprocess for making open-pore polyurethane structures containing powderedmetal comprising coherent spherical particles separated byinterconnected interstices.

Another object of the invention is to provide openpore polyurethanestructures containing powdered metal comprising coherent sphericalparticles separated by interconnected interstices.

LII

ecu-@011 NCO CH2 NCO wherein n has an average value of 0.5-2.0,containing about 4050' percent diisocyanate, the balance being tri-,tetraand pentaisocyanates, having a functionality of at least 3.0, ininert organic liquid diluents which form a homogeneous mixture in whichthe polyurethane produced herewith is substantially insoluble, (h)mixing the solutions together with metal powder, and ceasing said mixingbefore the onset of gelation, (0-) thereafter maintaining said mixturein a quiescent state while the polyurethane solution gels, and (d)removing said organic liquid.

By functionality of the polyisocyanate is meant the average number ofNCO groups per molecule. The isocyanate groups are convenientlydetermined by the amine equivalent method (ASTM D-l63867T). The hydroxylgroups of the polyol are determined by appropriate methods (ASTMD-l638-67T) and usually reported as hydroxyl number", i.e. the number ofmilligrams of potassium hydroxide equivalent to the hydroxyl contentoflgram of the sample.

By gelation" is'meant the change of state from the original usuallyclear solution in the absence of the metal powder to a gel or jelly,usually opaque. It may be detected by suitable viscosity measurements onsegregated portions of the mixture, as with a Brookfield rotationalviscometer, whereby a sharply rising viscosity indicates the onset ofgelation.

The process of this invention depends upon the relatively slowprecipitation of a polyurethane from a quiescent homogeneous dilutedmixture of the reactants. The following features are therefore critical;(a) the organic liquid diluent must serve as a nonsolvent for thepolyurethane product, (1)) the liquid diluent, or its components if amixture, must be a suitable inert solvent for the reactants; and (c) thereactivity of the polyurethane-forming reactants must not be so greatthat precipitation of the polyurethane occurs before the mixture attainsquiescence.

The organic liquid diluent may be selected from a wide variety of knownmaterials which are unreactive toward isoeyanates or polyols, e.g.,hydrocarbons including pentane, cyclopentane, hexane, cyclohexane,nonane; aromatic hydrocarbons including benzene, toluene, xylene, ethylbenzene, mesitylene, etc.; perfluoro compounds, includingperfluoroheptane, perfluorobenzene. etc.; halogen compounds, includingchloroform, carbon tetrachloride, l,l,l-trichloroethane. butyl chloride,etc.; ketones. including acetone, methyl ethyl ketone. diethyl ketone.etcx, ethers. including diethyl ether. B.B'-dichloroethyl ether.dioxane, tetrahydrofurane. etc.; esters. including ethyl formate. ethylacetate. butyl propionate, amyl butyrate. ethyl benzoate. etc.. amides.including nitro compounds. including nitroethane. nitropropane,nitrobenzene. etc.; and .sulfur compounds, including dimethyl sulfide.diethyl sulfide. dimethyl sulfone, dimethyl sulfoxide. etc. The lowerboil ing organic compounds are preferred since they can be most readilyremoved by evaporation.

The organic liquid diluent should be one in which the polyurethane issubstantially insoluble. Single liquids may be used, e.g.. toluene, ormixtures of liquids. e.g.. toluene with benzene, cyclohexane.tetrachloroethane. etc. The selection of diluents may be based on theSolubility Parameter Concept." The solubility parameter. 6. of eachliquid is a characteristic constant defined as the square root of thecohesive energy density (cf. J. L. Gordon, J. Paint Tech. 38, 43(1966)). For benzene. 8 is 9.15; for toluene, 8.9. etc. Furthermore, twoliquids having widely differing 8 values may be mixed in suitableproportions to yield mixtures having acceptable or even superior solventproperties. To be nonsolvents for the polyurethane polymers included inthe present in vention, the solubility parameter of the organic liquidor mixture of liquids is preferably in the range 8.5-9.0. It isessential that the higher molecular weight polyurethane polymers beinsoluble and precipitated in the organic liquid.

For simplicity it is desirable that the organic liquid diluent be asolvent for both types of reactants. The same liquid may then be usedfor both reactants. After the respective solutions have been prepared.mixed. and reacted, the organic liquid is readily recovered withoutcostly separation. However, different liquids may be used for therespective reactants provided the resulting solutions can be combined toyield a homogeneous mixture.

The reactivity of the polyisocyanate and the polyol should generally besuch that gelation of the polyisocyanate-polyol-organic liquid systemoccurs in a range of 5-60 minutes and preferably in 8-30 minutes. Tooshort a gelation time is apt to result in a weakened structure becausecondensation occurs before the system has reached a quiescent state;furthermore. shrinkage may be excessive. Too long a gelation time isunfavorable from a commerical and economic standpoint. The reactivity ofthe polyisocyanate and the polyol is related to a number of factorsamong which the most important are: their structure and the presence ofsubstituent groups such as hydrocarbyl. halo, nitro, etc.

As the preferred polyisocyanate there is employed a mixture of polyarylpolyalkylene polyisocyanates having the formula NCO wherein n has anaverage value of 0.5-2.0, containing about 40-50 percent diisocyanate,the balance being tri-, tetraand pentaisocyanates. having afunctionality of about 2.1-3.5. Examplesof other presently usefulpolyisocyanates are: cyclohexylene-l,4-diisocyanate;2.2-diphenylpropane-4.4-diisocyanate; 3,3-dimethyldiphenylmethane-4,4-diisocyanate; 1,4- naphthalene diisocyanate;l.5-naphthalene diisocyanate; diphenyl-4.4'-diisocyanate; 4.4,4-triphenylmethane triisocyanate; and 4.4',4",4"'- tetraphenylmethanetetraisocyanate.

Examples of polyols which may be employed with the polyisoeyanates are:glycerine. sorbitol, pentaerythriml, and the ethylene and propyleneoxide adducts of polyfunctional active-hydrogen compounds, such asglycerine. sorbitol, pentaerythritol, sucrose. trimethylolpropane. etc..having a functionality of at least 3.0. Preferred are the nitrogen-basedpolyether polyols obtained by totally oxypropylating an amine selectedfrom the group consisting of amines having the formula NH R-NH. where Ris an alkylene radical containing from 2 to 6 carbon atoms and amineshaving the formula where x is an integer of from 2 to 3, and y in aninteger of from 1 to 3. For example, N.N,N,N-tetrakis(2- hydroxypropyl)ethylenediamine. the polyoxypropylene derivatives of1,3-propanediamine, 1.4-butanediamine, 2.3-butanediamine.1.3-pentanediamine. 1.5- pentanediamine. 1.2-hexanediamine, 1,6-hexanediamine, diethylenetriamine, triethylenetetramine.tetraethylenepentamine. dipropylenetriamine. etc. As further examples ofthe preferred polyols, are the polyol obtained by totally oxypropylatingethylenediamine having a molecular weight of about 275-300 and ahydroxyl number of about 750-800; and the polyol obtained by totallyoxypropylating diethylenetriamine having a molecular weight of 400-600and a hydroxyl number of about 450-800. It is preferred that thehydroxyl functionality of the polyol be at least 4.0. Suitable materialshave been described in US. Pat. Nos. 2,626.9l5-l9 and 2,697,118.

Other factors influencing the reactivity of the system are the presenceof catalysts, e.g., tertiary amines, metal compounds. etc.; the natureof the solvent; the concentration of reactants in the solvent; theNCO/OH ratio of the system; and the temperature. If a given system hastoo short a gelation time, the above factors can be varied ascompensation. Thus. the temperature may be lowered or the catalysts maybe removed or neutralized. lfgelation time is too long. conversely thetemperature may be raised or catalysts added.

As catalysts there may be used accelerators for the reactions betweenpolyisocyanates and the polyols, c.g.. amines includingN-methylmorpholine. triethylamine, triethylenediamine. etc.. tincompounds including stannous chloride. tri-n-butyltin acetonate,di-nbutyltin diacetate, dimethyltin dichloride. etc. and othersincluding ferric acetylacetonate. The catalyst may be present in verysmall proportions, e.g., in quantities of from 0.005 to 0.5 percent byweight of the total mix.

Generally, in preparing the porous polyurethane structure, according tothis invention. solutions of the polyol and the dior polyisocyanate areprepared-separately in one or more organic liquid diluents, then mixedwith the powdered metal, poured into'a mold or onto a surface andallowed to stand in a quiescent state while the polymeric structure isforming. However, when either the polyisocyanate or the polyol is aliquid, it may be added with a limited amount of stirring into theorganic liquid diluent to which the other reactant has already beenadded and the powdered metal, then left standing undisturbed until set.The reactants, once mixed, quickly begin to react, and shortlythereafter, depending upon the temperature, solids content, catalyst,etc. form a gel which is left undisturbed until the structure has set.The point in time at which gelation occurs is reproducible for a givenset of conditions and may be easily determined by experimentation It isessential for the formation of the porous structures that no stirring bedone after this point. In prior art teachings, continuous stirring ofpolyurethane reactants in organic liquids has yielded either solutionsof elastomers and film-forming polymers, or precipitates of particulate,granular resins. neither having the structures of the present products.

As an explanation for the unexpected results of the precipitationprocess disclosed herein, it is suggested that the following operationsoccur. It is not known with certainty whether they actually occur inthis manner and whether they proceed in stepwise or continuous fashion.First, it is believed that the polyisocyanate and polyol reactantsinteract to form liquid-soluble,

short chain polymers. As the polymerization proceeds, the chain lengthsand molecular weights increase, until the polymeric material is nolonger soluble and acquires gel-like properties, i.e., is semi-dispersedin a swollen phase. Finally, as further reaction-at the ends of thepolymer chains yields even higher molecular weight material, thismaterial isprecipitated in situ.

The freshly formed surfaces have excellent cohesion so that there areformed aggregated coherent, roughly spherical particles which sticktogether in an interconnected matrix, As a consequence, there is formedan open network of polymeric material, having the organic liquid trappedwithin the polymer. The enmeshed liquid is thereafter readily removed'byevaporation orvolatilization under reduced pressure.

The concentration of reacting solids in the mixture can be controlled'bysimply changing the amount of organic liquid which is present.Preferably the concentration should be between to percent solids byweight. If the concentration is appreciablyless than l5 percent, thepolyurethane matrix will be weak and fragile; if more than 30 percent,the gels will tend to split and crack so that poor structural propertiesresult. Within limits, however, changing the concentration is a means ofchanging the density and porosity: the lower concentrations yield lessdense and more porous products.

The reaction yielding the polyurethane is preferably done at roomtemperature, although somewhat higher or lower temperatures may beemployed. Lower temperatures generally give less rigid structures, andhigher temperatures are undesirable if convection currents become severeenough to disturb the setting gel. The polyurethane matrix when freed oforganic liquid, may be further cured at moderate temperatures, e.g., 90to 150C. to remove odors or promote dimensional stability.

because of the novelprecipitation process by which thesestructures arefformed, they have 100 percent openpore construction. Any one pore isfreely communicating with another po re. The openings in the structureare irregular in shape. Neither in their appearance, nor in theirproperties, nor in their mode of formation do they resemble'the cellularfoams known in the art.

Porous metal structures with varying degree of bulk densities and poresizes were produced and evaluated. A wide range of strengths was foundto be achievable in the finished products by controlling the initialparticle size. of the filler material. Pore size, as would be expected,was affected by the size and shape of the initial structure. Prior tosintering it is burned out ofthe composition.

In order to form the final product, the metal powder formed bythe'p'recipitation techniques is sintered at an appropriate temperature.The length of sintering time affects the final filled product. Thiseffect was best demonstrated in the lighter and smaller diameter powdersof nickel. In most cases, extreme oxidation occurs upon sintering. Oncertain metal filled products, a limital oxidation seems to occur. Thiswasshown in the foam produced using the 25 micron nickel powder. Smallerdiameter nickel powders 'on the order of 3-5 microns seemed to'result'in a totally oxidized product, both inside and outside. The porosity ofthe products could be determined by the length of sintering.'Pore sizebecame smaller as oxidation was increased. This' was the result of theformation of the" nickel oxide which filled in the pore voldme. Thefinal pore'size limit was reached as the nickel became completelyoxidized. i

The strength of the porous, open-celled metal products approached 29,000p.s.i. on the totally sintered product using the 25 micron nickelpowder. This could be compared to the precursor urethane product which ihad a strength of 6.4 psi. at 10 percent compression. Pre-sinteredproduct which, in essence, consisted of nickel-filled polyurethaneranged in strength from approximately 60 psi. to 280 psi. incompression. All of the presintered products, however, were strongenough to hold their shape under lirnited physical stress. Thus thematerial could be handled readily without fear of crumbling or fallingapart.

Porosity ranged from a low of approximately 8 percent on the finishedsintered product of 2 5-micron nickel to a high of 92 percent on apartially sintered nickel product of 3-micron powder.

A number of nickel-filled products (polyurethane removed) were madehaving a wide range of strengths and physical properties. Theinterrelationships of these properties are shown in the FIGURE. Byvarying the concentration and the amounts of the initial filler powderit is possible to change the physical characteristics such as strength,pore size, and bulk density.

The process for making open-cell nickel structures, as described herein,is also applicable to the manufacture of any types of porous metalstructures, e.g. those made from powdered iron. aluminum. copper.silver.

ucts were open-pore polyurethanes as described in copending applicationSer. No. 828.647. filed May 28, 1969.

Compression tests were run on one-inch cubes of the gold. uranium. etc.products and were found to have strengths of 6.4 p.s.i. at percentdeflection with a compression recovery BRIEF DESCRIPTION OF DRAWING from75 to 88 percent (see Table 1). Bulk densities The FIGURE is a graph ofthe mechanical properties were also very low, ranging from 0.12 to 0.15g./cc.

Table 1 PHYSlCAL PROPERTY DATA OF PREClPlTATED POLYURETHANE (WlTHOUTMETAL) Yield Fail Yield Fail Strength Bulk Barcol Run Point PointStrength Strength at 10% del Density Porosity Hardness No. '71 Del /1Del lbs/in lbs/in lbs/in glee '7: air \'ol top/bot Remarks 1 89.1 25Ctemp o1 reaction 2 92.0 25C temp of reaction 3 6.35 .15 89.3 0C temp ofreaction 3-1.5 lbs/in at 5071 def recovery 75% 6.40 .15 90.6 0C temp ofreaction 23.5 lbs/in at 40% def recovery 5 .12 92.6

of porous nickel with the polyurethane burned off and EXAMPLE 2sintered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is furtherillustrated by. but not limited to, the following examples.

EXAMPLE 1 Precipitated Open-Pore Polyurethane (Without Metal) 102 gramsof LA-475 (ethylene diamine propylene oxide pentol) and 116 grams ofMondur MR (crude 4,4-diphenyl) methane diisocyanate) were reacted in thepresence of 2000 g. of toluene at various to 37C.) temperatures. Thereactants were allowed to sit without stirring in a liquid-confiningcontainer (any size or shape) unit] the polymerization was complete. Thesolvent was then readily removed from the polymerized matrix byevaporation in air at ambient temperature. Reaction temperatures of20C., 0C.. C. and 37C. were utilized. However. no differences in productappearance were noted with temoerature except that the material did notreact at 20C. The prod- Precipitated Open-Pore Polyurethane Filled With3-Micron Nickel Powder Mond 255 is a spherical nickel metal powder withan average diameter of 3 microns and a bulk density of about 1.32 g./cc.Using this particular product. metalfilled materials with bulk densitiesaround 1 g./cc. were formed. The nature of these materials is describedin Table 2.

To make the metal products described in Table 2. the two polyurethanereactants in the same amounts as Example 1 weFe' mixed together at roomtemperature (25C.) and about 2480 g. of Mond 255 Ni powder was addedslowly to the reactants, thoroughly mixed. and allowed to settle. Ahighly purified form 01 Mondur MR,E250, was utilized as the primaryreactant. The containers were covered and allowed to stand. (Glasstubing having diameters around seven-eighths in. and one-half in. wereused as containers to provide oneinch-long test cylinders forcompression strength measurement.) After the reaction was complete, thesamples were removed from the molds and air-dried in a hood to removethe excess solvent.

Table 2 PHYSICAL PROPERTY DATA ON 3 MICRON NICKEL FILLED POLYURETHANEDegree Yield Fail Yield Fail Strength Bulk Barcol Run of Point PointStrength Strength at 1071 del Density Porosity Hardness No. Sinter 71def 71 del lbs/in lbs/in lbs/in g/ec "/l air vol top bot Remarks 6-1none 1.11 8313 0 0 6-2 partial 1.20 87.8 0 0 6-3 total 1.47 71.0 0 (1 7none 97.8 1.15 90.2 0 (l 8 none 4.11 73.1 6-1.8 l .(12 88.7 (1 0 9 none24.1) 53.0 63.1 .878 89.9 (1 (l 10 none 63.1 .967 88.7 0 0 71.4 lbs/inat 2521 deflection 1 1 none 1.09 87.4 0 (1 12-1 none .993 87.4 0 0 12-2total 4.0 6.0 126.8 137.8 1.10 8 9 (l (1 13-1 none 1.11 87 (1 0 (1 1Sample lost when it melted on induction coil Table 2-Continued PHYSICALPROPERTY DATA ON 3 MICRON NICKEL FILLED POLYURETHANE Degree Yield FailYield Fail Strength Bulk Barcol Run of Point Point Strength Strength at10% def Density Porosity Hardness No. Sinter /z def /1 def lbs/inlhs/in' lhs/in" g/cc 71 air \01 top bot Remarks 14-1 none .975 87.6 0 014-2 total 2.0 193.1 1.40 82.8 0 0 15-1 none .920 87.7 0 0 15-2 total 15 5.0 127.4 176.6 1.21 85.7 0 0 16-1 none .922 88.1 0 0 16-2 partial 6.5152.5 156.4 1.09 91.8 0 0 17-1 none .979 87.3 0 0 17-2 partial 4 0 154.4169.9 1.15 90.0 0 0 18-1 none 1.00 87.0 0 0 18-2 partial 4.0 166.0 1.1989.4 0 0 19-1 none 1.20 84.1 0 0 19-2 partial 1.34 88.0 0 0 20 none 1.11 86.2 0 0 21-1 none 1.05 85.8 0 0 21-2 partial 1.24 88,7 0 0 21-3 total1.50 77.5 0 0 Products with hard and brittle crusts were noted. Allsamples had this crust, both from the large and small diameter tubes,thereby making the materials easy to handle and helping to retain shapeand prevent crumbling. The crust, approximately one-sixtyfourth-in. -in.in thickness, prevented powdering. From the compression data in Table 2it can be seen that the nonsintered products had sufficient strengths(about 100 p.s.i. at 10 percent compression) for routine handling and avery porosity (from 85 to 90 percent). When compressiontested, thematerial showed some degree of recovery, but the rigid crust preventedtotal material recovery.

Prior to sintering, the samples were placed in a muffle furnace at 300C.to burn off the polyurethane binder. The samples initially started tosmoke. A short time (10-20 seconds) later a red glow started from theends of the samples and proceeded until the whole sample was red.Shortly thereafter the glow disappeared. The products were removed andobserved. Slight distortion of shape was noted. The product was hard andblack and lost its compressibility. Some of these samples were retainedand compression-tested. These are listed under partial in the sinteringcolumn in Table 2.

The partially sintered materials exhibited very slight weight loss andno visible nickel monoxide formation, which is normally shown by a greenappearance. The samples retained a bulk density of approximately 1 g/ccand a porosity of around 88 percent. The partially sintered products hadcompression strengths of around 175 p.s.i. at failure (4 to 5 percentcompression). One sample had a compression strength of 156 p.s.i. at a10 percent compression. The crust which was found on the samples wasremoved from some samples before burning off of the polymer. Nodifferences in appearance were noted.

Final sintering was conducted in an oven at 500C, well below the normalsintering temperature. The samples seem to oxidize almost immediately.They started to glow a cherry red at the edges and the edges and theglow proceeded throughout the samples. The glow subsided in about 1.5min., after which the samples were removed and observed to be totallygreen in color. Run Nos. 6-3 and 21-3 cxibited no changes in weight.color or porosity after additional heating. 1t was assumed that thesesamples were totally oxided. The samples also showed no further changein shape or size. Compression tests showed a slight elevation incompressive strength, but all the samples tested failed and no recoverywas evident. These totally sintered samples retained the same bulkdensities and porosities as the partially sintered samples (Table 2). I

The partially sintered samples retained the ability to conduct acurrent; the totally sintered product could not conduct an electriccurrent.

One sample, Run 17. was heated on the induction coil to determine thefeasibility of sintering in this manner. The sample sintered. then.distorted and melted. Greater control of the technique is needed withthis particular product. but it is apparent that the technique isfeasible and useful.

The sintering can be carried out in an inert or reducing atmosphere suchas hydrogen. nitrogen, argon. helium or the like to prevent oxidation ofthe metal, if desired.

EXAMPLE 3 Metal Precipitated Open-Pore Polyurethane Filled With 5 MicronNickel Powder Mond 100 spherical nickel powder was used. This powder hasan average diameter of 5 microns and a bulk density of 3.0 g./cc.

The method of producing these products is the same as for Example 2using the same amounts of reactants except that about 5750 g. of the 5micron nickel was used in each run. N0 crust was evident and theproducts powdered slightly to the touch. The unsintercd product, asshown in Table 3, had compressive fail strengths from 60 p.s.i. top.s.i. at 4.5 percent compression.

During the burn-off the filled material exhibited none of thecharacteristics shown in the Mond 255 samples. At 300C. the samplesmerely smoked. There was slight distortion of shape at this point and avery slight weight loss from the loss of the urethane binder. After 22.5hours of sintering at 600C. the samples were removed. No furtherdistortion of samples was noted. The results of Example 3 are set forthin Table 3.

Table 3 PHYSICAL PROPER'IY DATA ON 5 MICRON NICKIiI. FILLEDPOLYURETHANE-l BASED FOAMS Degree Yield Fail Yield Fail Strength BulkBarcol Ran o1 Point Point Strength Strength at def Density PorosityHardness No. Sinter 91 def 71 def lbs/in lbs/in lbs/in g/cc 7: air voltop bot Remarks 22 total 2.52 65 61 61 23 none 3.5 68.3 2.63 0 0 24 none5.0 80.0 2.44 0 0 25 none 4.5 90.0 2.47 0 0 "6 none 5.5 60.0 2.31 0 0 27none 5.5 66.7 2.36 0 0 28 total 983.3 2.52 29 total 1.7 4377.5 3.58 49.790 60 30 total 8.5 5421.7 3.84 49.4 70 83 31 total 2.0 3975.9 3.67 50.982 65 32 total 3.25 58.0 70 15 33 total 2.2 6385.5 3.92 47.1 90 90 34total 3.27 57.2 92 90 35 total 3.18 57.2 80 65 36 total 5.0 1278.4 4.0740.8 93 60 Barcol hardness on side was 50 37 total .2 1562.5 3.18 54.990 0 38 total 4.0 8645.8 4.06 43.6 88 80 used in each run. When removedfrom the molds no crust was evident and the products powdered slightlyto the touch. The unsintered products had bulk densities of 4.5 g./cc.with porosities of about 51 percent. The nature of these products isdescribed in Table 4. The unsintered product has compressive failstrengths of 171 p.s.i. at a 2 percent fail to 284 p.s.i. at 1.5 percentfail. with an average of about 200 p.s.i. at 1.5 percent fail.

Table 4 PHYSICAL PROPERTY DATA ON 25 MICRON NICKEL FILLED POLYURETHANEDegree Yield Fail Yield Fail Strength Bulk Barcol Run of Point PointStrength Strength at 107z del Density Porosity Hardness No. Sinter 7!def def lbs/in lbs/in lbs/in g/cc 71 air vol top bot Remarks 39 1 none4.39 55.0 0 0 39-2 partial 4.34 54.0 75 75 40 none 2.0 171.1 4.26 54.2 00 41. none 1.5 249.2 4.53 50.8 0 0 42 none 1.0 196.0 4.51 51.6 0 0 43none 1.5 284.4 5.12 47.5 0 0 44-1 none 4.0 220.0 4.78 50.3 0 0 44-2total none none 5.48 16.2 98 98 17.241 lbs/in 10092 recovery 45 total6.5 16.379 5.08 18.7 98 98 46 total none none 5.58 14.9 100 100 17.241lbs/in 100% recovery 47 total 5.88 12.6 100 100 48 total none none13.595 5.79 15.0 100 100 1007: recovery 49 total none none 14.132 5.8215.9 100 100 100C: recovery 50 total 5.0 28.700 5.47 25.4 100 100 51total none none 6.33 7.7 100 100 36.101 Ibs/in" at 6.5 2 10092 recovery52 total ;l1(\l10 none 27.797 5.76 20.1 100 100 100G recoverv 53 1011114.0 22.744 5.16 27.x 100 100 54 total 5.80 20.8 99 99 sintered foamswere nonconductive because of their composition of nickel oxide.

The appearance-0f the sintered materials was that of a fully oxidized(N103) product. green throughout.

EXAMPLE 4 Precipitated Open-Pore Polyurethane Filled With 25-MicronNickel Powder During burn-off of the filled polyurethane at 300C.nothing was observed except smoking. No distortion or change of shapeoccurred with the heating of the samples. After total sintering nochanges had occurred except a dull green color that did not resemble thefull green of the previous products. When scratched. the surface lookedlike nickel metal but the materials would not conduct electricity.

The compressive strengths of the samples after sintering were very high.Some of the specimens broke under the load and others did not. Threesamples tested to a 10 percent compression; three samples tested overthe test cell limits (10.000 pounds) at approximately 6.5 percentcompressions; and three samples actually broke under the testing loadbetween 4 and 6.5 percent compression. The samples which did not breakor fail had 100 percent compression recovery. Compression strengthsranged from 13,500 p.s.i. (72 deflection) to 36,000 p.s.i. (6.5%deflection).

The bulk densities of the sintered samples ranged from 5.0 g./cc. to 6.3g./cc. Thc porosities ranged from 7.7 percent of a high of 25.4 percentopen space, with an average of about 15 percent. All the samples. eventhe smallest porosity samples, were able to absorb water but the higherthe porosity the lower absorption time. The sintered products testedfavorably with the Barcol hardness tester. All of the samples rangedfrom 98 to 100. Eight out of ll samples showed 100 on the hardnessscale.

Other samples were heated by using the RF induction heating coil.Apparently, full sintering did occur but it was an uneven type ofsintering. The outside portions of the product appeared to be morehighly oxidized than the center. Heating on these samples occurred sorapidly that the shape was distorted and cracks appeared. One sample infact was conductive to electricity in the center portion andnon-conductive in the outside surface or periphery. One sample was toolarge for the coil to handle and it could not be sintered. This methodof sintering should be conducted under more closely controlledconditions.

What is claimed is:

l. A porous metal structure made by heating at a sufficient temperatureto remove the polyurethane an open-pore polyurethane structurecontaining powdered metal dispersed in said polyurethane, said structurecomprising coherent, roughly spherical polyurethane particles containingsaid powdered metal, said spherical particles sticking together in aninterconnectcd matrix. and in which the polyurethane is the product ofthe reaction between a. a mixture of polyaryl polyalkylenepolyisocyanates having the formula CH NCO where is an integer of from 2to 3, and y is an integer of from 1 to 3.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,897,221 Dated July 29, 1975 ifii rentofls) Iva]. O. Salyer, et a1 Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

The term of this patent subsequent to May 7, 1989, has been disclaimed.

Signed and Scaled this v Sixth vDay of July 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Atresling Officer (ummis-sinner ofParemsand Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3,897, 221 Dated July 29, 1975 im re ntofls) Ival O. Salyer,et a1 It is certified that error appears in the above-identified patentarid that said Letters Patent are hereby corrected as shown below:

The term of this patent subsequent to May 7, 1989, has been disclaimed.

V I V 'Signcd and gcalcd this Sixth Day of July 1976 [SEAL] AIIeSI.

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Parenlsand Trademarks

1. A POROUS METAL STRUCTURE MADE BY HEATING AT A SUFFICIENT TEMPERATURETO REMOVE THE POLYURETHANE AN OPEN-PORE POLYURETHANE STRUCTURECONTAINING POWDERED METAL DISPERED IN SAID POLYURETHANE, SAID STRUCTURECOMPRISING COHERENT, ROUGHLY SPHERICCAL POLYURETHANE PARTICLESCONTAINING SAID POWDERED METAL, SAID SPHERICAL PARTICLES STICKINGTOGETHER IN AN INTERCONNECTED MATRIX, AND IN WHICH THE POLYURETHANE ISTHE PRODUCT OF THE REACTION BETWEEN A. A MIXTURE OF POLYARYLPOLYALKYLENE POLYISOCYANATES HAVING THE FORMULA